The quick answer: 9.5dB at 1300nm for LX/LX GBIC, and 23dB at 1550nm for ZX GBIC. Stay under these numbers for total attenuation end-to-end, and your link will be 100% reliable. Exceed them, the link may work, but don't count on it.
At short distances, no minimum attenuation required for connecting LX/LH GBICs point-to-point; just a minimum distance patch cable, length 2 meters.
ZX GBICs require at least 7.77 or 8dB total attenuation in the SMF span, to avoid overdriving the RX optics. On short runs, especially labs or test configurations where you just use a 2m patch cable, you need to install Inline attenuators of at least 8dB. These go directly on the RX ports of each GBIC. As the length of the span increases, so does natural cable attenuation and splice losses, which decreases the amount of inline attenuation needed. At 40km or 50km and beyond (depending on which Cisco source you read), inline attenuation is not required.
Hope this helps.
